Have you tried Wordle?

Wordle is a toy for generating “word clouds” from text that you provide. The clouds give greater prominence to words that appear more frequently in the source text.

You can use any text, a rss feed or a del.icio.us username to create a “word cloud”, after that it’s possible to tweak colours, size, number of words and a bunch of other options.

It’s also possible to see the “word clouds” from other people. Great idea! Brilliant execution.

Here’s an example from my blog feed: